With a greatly expanding use of plastics materials as a substitute for metals in the manufacture of housings, containers and components of machines there arises a prevalence of polymer based fires when such components are ignited thus providing an added danger.
Fire retardants can be added to plastics materials but the service life of those retardants is often substantially less than the service life of the plastics component. Typical fire retardants are relatively volatile and vaporise from the plastics material over time. When plastics materials are exposed to heat they can ignite and/or melt releasing toxic volatiles and exhibiting problems associated with polymer based fires.
A further complicating aspect of this problem is the difficulty in detecting the presence of a fire. Various types of sensors and detectors are available for detecting the presence of a fire. However, the applicant required the presence of a fire to be detected in an environment of relatively high velocity air currents. This type of environment is predominant in air conditioning housings and in particular evaporative air coolers. As this type of cooler depends upon the generation of high velocity air currents for the supply of large volumes of air, it presents a difficulty for the detection of fire.
Various detectors and sensors are available for detecting fires which are considered to be the most appropriate for this type of environment such as infra red sensors, ultra violet sensors and visible light sensors. In addition, there are also commercially available thermal detectors. However, the applicant discovered significant disadvantages with each of the various types of detectors generally recommended for the target environment. These disadvantages were primarily the relatively high cost for the various types of radiation detectors and the restricted scope of detection of thermal detectors (i.e. detection only for regions in close proximity of the detector). Additionally, various problems were encountered with the use of radiation detectors in relation to false alarm due to leakage of outside light into the housing of the air conditioners. Also, to provide adequate coverage of the entire unit to be protected, numerous of these detectors were required which adds significantly to the cost of the apparatus.
Whilst one of the simplest and probably the least expensive type of detector for the detection of a fire is a standard domestic smoke detector, the applicant soon discovered that this type of detector was not able to be employed successfully in environments comprising high velocity air currents. The difficulty of detecting the presence of small quantities of smoke in a fast moving air stream is well known, and numerous devices have been developed in attempts to address this problem.
Use of all types of smoke detectors proved to be ineffective as the smoke particles were so diluted in the air stream that they did not occur in sufficient concentration to insure effective detection by the smoke detector. In addition, the high velocity air currents also act to degrade the sensitivity of the smoke detectors.
Various configurations of smoke detectors were attempted and tested with smoke sources including the positioning of numerous detectors in and around various locations of the air conditioner and the installation. In one configuration, a smoke detector was installed in the housing of the cooler with a second detector located downstream within one of the air conditioning ducts. Even with this configuration, the dilution of smoke particles in the ducting was sufficient to render the smoke detector incapable of detecting their presence in sufficient concentration.